Safety and immunogenicity of a third dose of COVID-19 protein subunit vaccine (Covovax(TM)) after homologous and heterologous two-dose regimens

OBJECTIVES: To report the safety and immunogenicity profile of a protein subunit vaccine (Covovax(TM)) given as a third (booster) dose to individuals primed with different primary vaccine regimens. METHODS: A third dose was administered to individuals with an interval range of 3-10 months after the second dose. The four groups were classified according to their primary vaccine regimens, including two-dose BBIBP-CorV, AZD1222, BNT162b2, and CoronaVac/AZD1222. Immunogenicity analysis was performed to determine binding antibodies, neutralizing activity, and the T-cell responses. RESULTS: Overall, 210 individuals were enrolled and boosted with the Covovax(TM) vaccine. The reactogenicity was mild to moderate. Most participants elicited a high level of binding and neutralizing antibody against Wild-type and Omicron variants after the booster dose. In participants who were antinucleocapsid immunoglobulin G-negative from all groups, a booster dose could elicit neutralizing activity to Wild-type and Omicron variants by more than 95% and 70% inhibition at 28 days, respectively. The Covovax(TM) vaccine could elicit a cell-mediated immune response. CONCLUSION: The protein subunit vaccine (Covovax(TM)) can be proposed as a booster dose after two different priming dose regimens. It has strong immunogenicity and good safety profiles.